The free world is the new continent in cyberspace that we have built so we can live here in freedom. It's impossible to live in freedom in the old world of cyberspace, where every program has its feudal lord that bullies and mistreats the users. So, to live in freedom we have to build a new continent. Because this is a virtual continent, it has room for everyone, and there are no immigration restrictions. - Richard Stallman -

... and make sure that the start mode is set to standalone and set VIRTUALCHROOT=true:

[...]
STANDALONE_OR_INETD=standalone
[...]
VIRTUALCHROOT=true
[...]

Now we configure PureFTPd to allow FTP and TLS sessions. FTP is a very insecure protocol because all passwords and all data are transferred in clear text. By using TLS, the whole communication can be encrypted, thus making FTP much more secure.

If you want to allow FTP and TLS sessions, run

echo 1 > /etc/pure-ftpd/conf/TLS

In order to use TLS, we must create an SSL certificate. I create it in /etc/ssl/private/, therefore I create that directory first:

SquirrelMail Configuration : Read: config.php
---------------------------------------------------------
While we have been building SquirrelMail, we have discovered some
preferences that work better with some servers that don't work so
well with others. If you select your IMAP server, this option will
set some pre-defined settings for that server.

Please note that you will still need to go through and make sure
everything is correct. This does not change everything. There are
only a few settings that this will change.

SquirrelMail Configuration : Read: config.php
---------------------------------------------------------
While we have been building SquirrelMail, we have discovered some
preferences that work better with some servers that don't work so
well with others. If you select your IMAP server, this option will
set some pre-defined settings for that server.

Please note that you will still need to go through and make sure
everything is correct. This does not change everything. There are
only a few settings that this will change.

Command >><-- Q
You can now find SquirrelMail in the /usr/share/squirrelmail/ directory.
After you have installed ISPConfig 3, you can access SquirrelMail as follows:
The ISPConfig apps vhost on port 8081 for nginx comes with a SquirrelMail configuration, so you can use http://server1.example.com:8081/squirrelmail or http://server1.example.com:8081/webmail to access SquirrelMail.

If you want to use a /webmail or /squirrelmail alias that you can use from your web sites, this is a bit more complicated than for Apache because nginx does not have global aliases (i.e., aliases that can be defined for all vhosts). Therefore you have to define these aliases for each vhost from which you want to access SquirrelMail.
To do this, paste the following into the nginx Directives field on the Options tab of the web site in ISPConfig:

If you use both http and https for your vhost, you need to add the following section to the http {} section in /etc/nginx/nginx.conf (before any include lines) which determines if the visitor uses http or https and sets the $fastcgi_https variable (which we will use in our SquirrelMail configuration) accordingly:

Then go to the nginx Directives field again, and instead of fastcgi_param HTTPS on; you add the line fastcgi_param HTTPS $fastcgi_https; so that you can use SquirrelMail for both http and https requests: